Method for bonding thermoplastic or thermoset polymeric materials utilizing voltage applied to conductive material

ABSTRACT

A method for bonding or joining thermoplastic or thermoset polymeric materials is provided which comprises applying a layer of conductive material ( 16 ) at a joint or between surfaces of thermoplastic or thermoset polymeric materials ( 12, 14 ), introducing a current so as to raise the temperature of the polymeric material above its melting point, maintaining the current for a time sufficient to allow the polymeric material to become molten and thereby form the joint or bond, and then removing the current and allowing the joint or bond ( 15 ) to cool. The method is advantageous in that it provides a simple and inexpensive means of bonding thermoplastic or thermoset polymeric materials with a minimum of materials and equipment The method of the present invention can also be utilized so as to heat thermoplastic pipes and other devices made of thermoplastic materials.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 60/346,277, filed Jan. 9, 2002.

FIELD OF THE INVENTION

This invention relates in general to a method for bonding thermoplasticor thermoset polymeric materials, and in particular to a method andproduct of said method wherein a conductive material such as a carbonvoile or a conductive paint or ink is applied to a joint surface or to alocation between two surfaces to be bonded, and an electrical current isapplied to the conductive material so as to heat the polymeric materialsabove their melting point and allow a bond to form between the twopolymeric surfaces.

BACKGROUND OF THE INVENTION

Joining two pieces or parts made of polymers is normally achieved usinga variety of methods of introducing energy into the joint face. Thesemethods include vibrating the two surfaces together, using ultra-sonicvibration to excite the material at the interface, using radio frequencyelectromagnetic energy to cause the interface to be heated, heating thetwo surfaces independently and bringing the surfaces together in amolten state, adding heat energy at the outside surfaces such that it isconducted to the interface. All of these methods use complex machineryand are generally inefficient and hard to use accurately, and thus arevery limited in their use.

There is thus a distinct need in the field to develop methods of bondingthermoplastic or thermoset polymeric materials devices which can besimple and inexpensive to use, which provides a rapid and precisethermal bond which is of excellent strength and durability, and whichcan be achieved with readily available and easy-to-incorporatematerials.

SUMMARY OF THE INVENTION

Accordingly, it is thus an object of the present invention to provide amethod for bonding thermoplastic or thermoset polymeric materialsutilizing conductive materials strategically located at a joint orbetween two polymeric surfaces to be bonded.

It is further an object of the present invention to provide a methodwherein induction of a suitable electrical current applies a voltage toconductive materials at a joint or other bonding surface which heats thethermoplastic materials to be bonded such that the materials becomemolten and form a joint or bond between the surfaces.

It is still further an object of the present invention to provide amethod wherein a carbon voile or a conductive paint or ink is applied toa joint or between sheets of thermoplastic material which willultimately be utilized to heat the materials and form the bond or jointwhen a voltage is created in the carbon voile or conductive paint orink.

It is even further an object of the present invention to provide anefficient and inexpensive method of forming a joint or bond in polymericmaterials using conductive carbon or other suitable conductivematerials, and to provide welded or joined thermoplastic materialsobtained by said method.

These and other objects are achieved by virtue of the present inventionwhich provides a method and product produced thereby which comprisesapplying a layer of conductive material at a joint or between surfacesof thermoplastic or thermoset polymeric materials to be bonded orjoined, introducing a suitable voltage or current to the conductivematerial so as to raise the temperature of the polymeric material aboveits melting point, maintaining the voltage or current for a timesufficient to allow the polymeric material to become molten and therebyform a joint or bond, and then removing the voltage or current so as toallow the joint or bond to cool. Utilizing the method of the presentinvention, a simple and inexpensive means is provided to allow bondingand joint formation of suitable thermoplastic or thermoset polymericmaterials in an efficient and precise manner. The method of the presentinvention can also be utilized so as to heat thermoplastic pipes andother devices made of thermoplastic materials.

These and other features of the present invention as set forth in, orwill become obvious from, the detailed description of the preferredembodiments provided hereinbelow.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 is a side schematic view of thermoplastic or thermoset polymericsheets positioned to be bonded using the method of the presentinvention.

FIG. 2 is a top view of the thermoplastic or thermoset polymeric sheetsof FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the present invention, there is provided a method forbonding or joining thermoplastic or thermoset polymeric materials whichin general comprises the steps of applying a layer of conductivematerial between two thermoplastic or thermoset polymeric surfaces to bebonded or formed into a joint; introducing a voltage to the conductivematerial so as to cause the surfaces of the polymeric material to heatto a temperature above their melting point and maintaining said voltageuntil such time as the surfaces of the polymeric materials become moltenand a bond or joint forms between the surfaces; and removing saidvoltage and allowing the bonded thermoplastic materials to cool to atemperature below their melting point so as to solidify said bond orjoint. As indicated in more detail below, the conductive materialutilized along with the thermoplastic or thermoset polymeric materialsmay be any suitable conductive material such as a carbon fiber, e.g., inthe form of a sheet or voile, or a suitable conductive paint or inkwhich may be applied to the thermoplastic surface via standard screenprinting techniques.

In accordance with the invention, the invention may be utilized alongwith any of the many known thermoplastic or thermoset polymericmaterials that would be familiar to those of ordinary skill in thepertinent art. For example, suitable polymeric materials would includesuch thermopolymers as polymers, copolymers or mixtures thereof whichcan be formed into suitable thermoplastic materials, e.g., into sheets,pipes, planters, etc., and such suitable polymeric materials can includepolyethylene, polypropylene and other polyolefins, polyethyleneterephthalate and other polyesters; polyamides; polyimides; polystyreneand other vinyl polymers, nylon, ABS and PC-ABS. One such sheet materialparticularly suited for use in the method of the present invention ishigh density polyethylene or HDPE. Still other suitable polymers aredisclosed in U.S. Pat. Nos. 4,594,203; 3,975,473; 3,988,401; and4,148,954, said patents incorporated herein by reference.

In the preferred embodiments, as set forth in more detail below, asuitable conductive material is employed between two thermoplastic orthermoset materials to be bonded or formed into a joint, and thismaterial is preferably applied either by coating at least one of the twosurfaces to be joined, or by inserting a conductive carbon fiber sheetor voile between the two polymeric materials to be joined. When appliedin the form of a conductive coating, the preferred method is to apply aconductive ink or paint to one of the two surfaces to be joined orbonded. Wherein the conductive ink or paint is applied, these may beapplied using any conventional application technique including standardscreen printing techniques. There are many suitable conductive inks andpaints that are well known in the field and which will be useful inaccordance with the present invention, and these include conductivepaints such as those disclosed in U.S. Pat. No. 4,499,010, incorporatedherein by reference, and conductive inks such as those disclosed in U.S.Pat. Nos. 4,369,269 and 4,443,495, also incorporated herein byreference. When using the carbon voile, it is preferred that the carbonvoile be inserted at the interface between the surfaces to be bonded,and it is preferred that that carbon voile have an approximate densityof less than 10 grams per square meter. It is also possible that thecarbon voile may be lightly pierced if so desired.

In either of these embodiments, it is also preferred that a conductivemetallic tape or wire be applied in a suitable manner so as to provide aconductive pathway for the application of current or voltage to theconductive material forming the bond. In both cases, the conductive tapeor wire may be applied along the opposite sides or ends of the carbonvoile or the conductive compound so that a current may be directed tothe bonding interface when necessary. The tape or wire may be applied ina manner so that it is removable once bonding is completed, or it may beof a material that will be compatible with the thermoplastic and thusbecome a permanent part of the joint or bond when completed. Inaddition, in the preferred process, the materials to be bonded or formedinto a joint are preferably clamped together in a suitable manner beforeapplying the bonding voltage, and such clamping may be obtained usingmechanical pressure, e.g., via a conventional clamp or other mechanicalmeans, and may also be obtained by adjustment of atmospheric pressure,e.g., by evacuation or drawing a vacuum on the site of joining orbonding to keep the surfaces of the polymeric materials in placethroughout the bonding process.

There are many suitable methods for carrying out the present inventionincluding those specific embodiments as described below. The mainfeature of the present invention is the utilization of the conductiveproperties of a suitable conductive material such as carbon, such as aconductive paint or ink or a carbon voile, which can introduce thermalenergy to a joint surface or to the interior of a thermoplastic orcomposite by incorporating the carbon fiber or conductive material(e.g., a conductive paint or ink) into the bonding location or joint andby applying a low voltage to the carbon material. The conductivematerial, e.g. carbon voile (at approximate density of 10 grams persquare meter or less) or conductive ink or paint, is introduced into ajoint or between sheets of the thermoplastic or thermoset material.Alternatively, the conductive material, in particular when in the formof a conductive ink or paint, may preferably be coated onto one or bothof the surfaces to be joined or bonded, and such coating may be appliedusing suitable techniques such as standard screen printing techniques.In either case, it is also preferred that a metallic conductive tape orwire be applied along the opposite sides or ends of the carbon voile orother conductive material, and such tape or wire is useful in applying avoltage suitable to heat the thermoplastic material above its meltingpoint until a bond or joint is formed in accordance with the invention.

As indicated above, the conductive material may comprise a welding tapemade up of a carbon fiber sheet or voile edged with a narrow conductivetape or band. The tape can also be furnished with adhesive properties sothat the joint is held in place while the weld occurs. The carbon sheetor voile is then preferably sandwiched between the two polymericsurfaces to be joined. In the alternative, the voile can be incorporatedinto a thermoset composite by placing the voile between two layers ofthermoset, preferably in the ‘tack’ stage of polymerization.Alternatively, the conductive material may comprise a conductivecompound in the form of a conductive paint or ink, and in this case, thecompound is applied and allowed to dry, after which a conductivemetallic strip, e.g., one with a self-adhesive on one or both sides, isapplied to the conductive material, preferably at the sides or ends. Ineither such instance, the voile or the conductive compound is bounded ontwo opposite edges with a conductive wire or tape, the ends of whichprotrude beyond the periphery of the completed sheet. As describedfurther below, when a low voltage is applied to the material through theconductive tapes or wires, the surface temperature of the sheet israised to the desired level and maintained at that level such that thesheet acts as a radiator. The invention can thus be used as a wallradiator, for heated floors or ceilings, or to make heated tanks orcontainers and in other applications.

It is also preferred that when forming the joint or bond, the polymericmaterials to be joined or bonded are clamped together using eithermechanical pressure or by evacuating the joint itself relying onatmospheric pressure to apply pressure (e.g., vacuum pressure) to thejoint. When clamped in suitable position to maintain an interfacebetween the surfaces to be bonded or joined, a voltage is applied, themagnitude of which is dependent on the length and width of the joint,but which will be suitable to raise the temperature of the polymericmaterials to above the melting point for a time sufficient to form thebond or joint. Thus, the voltage applied so as to cause a current ofsufficient strength to be applied to raise the temperature within thejoint to above the melt point of the plastic. At that point, the plasticflows through the fine interstices in the carbon voile, or through theconductive coating material, and a structural joint or bond is achieved.In the preferred process, the voltage applied is a low voltage, but thisvoltage should be suitable to melt the polymers and form the bond in arapid time period, even as little as ten seconds or less in theparticularly preferred embodiment.

The present invention is illustrated further in the accompanying drawingfigures which show suitable thermoplastic sheets or panels in positionfor the bonding method of the invention. As shown in the side view ofFIG. 1 and then top view of FIG. 2, the present invention allows theformation of a bond between two thermoplastic or thermoset polymericmaterials such as a top sheet 12 and a bottom sheet 14 which are to bejoined together in the zone of overlap, generally shown at 15. In theembodiment shown in the drawing figures, the conductive material placedbetween the two panels or sheets to achieve bonding is a conductivecarbon voile 16 which is inserted in sandwich fashion between top sheet12 and bottom sheet 14. In the embodiment illustrated in the drawingfigures, strips of conductive tape, 17 and 19, are placed in contactwith the carbon voile 16 and with sheets 12 and 14 so as to allow acurrent to be introduced into the carbon voile that will ultimatelycause heating and bonding between the sheets 12 and 14. In the preferredembodiment, the conductive tape is a metallic tape with adhesive fixedupon both sides so that the tape stays in contact with the carbon voileand the sheets during the bonding process, and the tapes are designed sothat one will act as a positive electrode and one will act as a negativeelectrode.

In the preferred process as described above, the sheets 12 and 14 may beclamped in place by any suitable means capable of applying mechanicalpressure, such as a clamp, a press or other suitable clamping device.Alternatively, as observed in the drawing figures, the area to be bondedmay be kept in place by means of a vacuum pressure, and thus in thisembodiment, adhesive tape 18 is used over the end of sheet 12 andconductive tape 17, and adhesive tape 20 is used over the end of sheet14 and conductive tape 19 so that the bonding area will hold a vacuumapplied by suitable evacuating means (not shown). The adhesive tapeelements 18 and 20 are preferably removed following the bondingprocedure.

In the preferred process of the invention, sheets 12 and 14 are bondedby applying an electrical current or voltage to voile 18 by means of anelectrical connection from a transformer or other suitable electricalsource (not shown), and the current is applied via the conductive tapeelements 17 and 19 so that panels 12 and 14 heat up above their meltingpoint and become molten so that a bond forms in the overlap region 15.Following formation of the bond, which can occur by administration ofsuitable voltage for as little as 10 seconds or less, the voltage isremoved from the system, and the bonded sheets 12 and 14 are allowed tocool down so as to solidify the bond. In the preferred process, adhesivetape elements 18 and 20 are removed following formation of the bond andwithdrawal of the vacuum pressure that originally kept the panels 12 and14 in place.

In terms of the application of voltage and the particular sizes andcompositions of the polymeric materials used in the invention, a widerange of polymers can be welded using the present method, and these haveemployed materials of different sizes and thickness. In general, foreach application, the general method is to establish the amount of heatenergy required to melt the plastic, allowing an average thickness to bemelted of some 1.0 mm or less. By determining the resistance of theconductive film or voile in the joint, the energy in joules/sec that isdelivered for a given voltage can be calculated. The time taken to weldthe joint can therefore be calculated.

The resistance of the conductive layer will be proportional to the widthof the conductive film and a formula is then used to determine thecombination of voltage and time best suited to the size and architectureof the joint.

In typical examples of the method of the invention, using polyolefinssuch as polypropylene or polyethylene, a joint line 12 mm wide and 100mm long may be welded in about 4.0 seconds, applying a voltage of about70 volts to the carbon voile. The time required is proportional to theapplied voltage, and as indicated above, the actual voltage applied inany particular case will be based at least in part on the conditions andthicknesses of the materials involved. In addition, where the conductivecompound is used, the resistance is lower and a higher voltage istherefore applied.

By utilizing the method of the present invention, one achieves benefitsnot heretofore possible using prior methods. These benefits of themethod include the ability to use simple and inexpensive conventionequipment, e.g., a variable transformer capable of adjusting the voltageto the desired level while fitted with appropriate conventional andreadily available safety devices such as a timer and an earth leakagecircuit breaker, and low cost metallic tape. Moreover, the method offersthe benefit of an extremely rapid welding process being achievedtypically in less than 10 seconds for a significant weld area and givinggood structural integrity, particularly where the weld area isreinforced by the carbon fibre voile material. In addition, othersignificant advantages of the present invention are obtained in that themolten thermoplastic is at all times protected from the atmosphere suchthat no oxidation takes place. A further advantage is that the outsidesurfaces of the joint are in no way affected by the weld and thereforedelicate materials and finishes can be welded without damage.

Even further advantages are realized in that the system can be used toweld materials containing reinforcing fibres where very high jointpressures can be applied to ensure that the joint retains the fullstrength of the parent materials. Because it is only the joint surfacesthat are heated, and because that heating may be achieved rapidly andmaintained only for a short period, very high pressures can be appliedto ensure that the strongest possible joint is obtained.

In other embodiments of the present invention, the present method andmaterials may also be utilized to bend thermoplastic or thermosetmaterials such as large sheets of thermoplastic. In this method, ingeneral, the method comprises applying a layer of conductive material asdescribed above to a suitable thermoplastic or thermoset polymericmaterial, introducing a voltage to the conductive material so as tocause the surface of the polymeric material to heat to a temperatureabove its melting point and form a bendable portion in the thermoplasticor thermoset polymeric material, bending said polymeric material to adesired angle; and removing said voltage after said desired bendingangle is achieved to allow the thermoplastic or thermoset polymericmaterial to harden at the desired angle. In the preferred process, thebending is carried out by incorporating a thin carbon sheet voile ortape, or by coating or printing a suitable strip of conductive material,such as the inks or paints described above, and in the particularlypreferred embodiment, this conductive material is applied with a widthequal to approximately ten times the thickness of the sheet. When a lowvoltage is applied to the voile or tape, or conductive compound asdescribed above, the material can be heated and as a result easilyformed into a bend of the desired angle and radius. In accordance withthe invention, the welding tape or sheet can also be used to heat pipes,valves and similar fittings to prevent the contents of the pipesfreezing or become viscid. In this application of the present invention,it is preferred that the tape be wrapped around the pipe in lengths ofnot more than 120 inches, with care being taken to leave a gap betweeneach wrapping of the tape around the pipe. The outside surface is thenwrapped with a thermoplastic or fabric tape, capable of withstanding theoperating temperature and a low voltage applied to the carbon tape endto end to raise and maintain the temperature of the pipe or fitting.

In yet another embodiment of the present invention, it is possible touse the conductive sheet or tape of the invention to heat a suitablethermoplastic planter, such as the “Earth Box®” which is disclosed inU.S. Trademark Registration No. 1,906,561, and in U.S. Pat. Nos.5,555,675; 5,524,387; 5,379,547; 5,193,306 and 5,103,584, said patentsand trademark registrations incorporated herein by reference). In thisprocess, a strip of polymer material in which is embedded a strip ofcarbon fibre voile (or a conductive coating as described above) isapplied or affixed to the inside or outward side of the Earth Box® Thestrip of polymer material would typically incorporate a backing sheet,between the strip and the side of the Earth Box®, of reflective foil.The strip would typically be maintained at a temperature above ambientby the application of a low voltage current in the manner as set forthabove.

It is thus submitted that the foregoing embodiments are onlyillustrative of the claimed invention and not limiting of the inventionin any way, and alternative embodiments that would be obvious to oneskilled in the art not specifically set forth above also fall within thescope of the claims.

The following example is presented as illustrative of the presentinvention or methods of carrying out the invention, and is not providedas restrictive or limiting of the scope of the invention in any manner.

EXAMPLE

A wide range of polymers have been welded using the method of thepresent invention, and these have employed materials of different sizesand thickness. In general, for each application, the general method isto establish the amount of heat energy required to melt the plastic,allowing an average thickness to be melted of some 1.Omm or less. Bydetermining the resistance of the conductive film or voile in the joint,the energy in joules/sec that is delivered for a given voltage can becalculated. The time taken to weld the joint can therefore becalculated.

The resistance of the conductive layer will be proportional to the widthof the conductive film and a formula is then used to determine thecombination of voltage and time best suited to the size and architectureof the joint.

In one specific example, using polyolefins such as polypropylene orpolyethylene, a joint line 12 mm wide and 100 mm long is welded in 4.0seconds, applying a voltage of 70 volts to the carbon voile. The timerequired is proportional to the applied voltage. Where the conductivecompound is used, the resistance is lower and a higher voltage istherefore applied.

1. A method for bonding or joining thermoplastic or thermoset polymericmaterials comprising: (a) applying a layer of conductive materialbetween two thermoplastic or thermoset polymeric surfaces to be bondedor formed into a joint; (b) introducing a voltage to the conductivematerial so as to cause the surfaces of the polymeric material to heatto a temperature above their melting point and maintaining said voltageuntil such time as the surfaces of the polymeric materials become moltenand a bond or joint forms between the surfaces; and (c) removing saidvoltage and allowing the bonded thermoplastic materials to cool to atemperature below their melting point so as to solidify said bond orjoint.
 2. The method of claim 1 wherein the two polymeric surfaces areclamped together before the introduction of a voltage.
 3. The method ofclaim 2 wherein the two polymeric surfaces are clamped using eithermechanical pressure or by evacuation to create a vacuum pressure at theinterface between the two surfaces.
 4. The method of claim 1 wherein theconductive material comprises a conductive carbon fiber in the form of asheet or voile.
 5. The method of claim 1 wherein the conductive materialcomprises a conductive ink or paint.
 6. The method of claim 5 whereinthe conductive ink or paint is applied to at least one of the twopolymeric surfaces to be bonded or formed into a joint.
 7. The method ofclaim 5 wherein the conductive ink or paint is applied using standardscreen printing techniques.
 8. The method of claim 1 wherein thethermoplastic or thermoset polymeric materials are selected from thegroup consisting of polyethylene, polypropylene and other polyolefins,polyethylene terephthalate and other polyesters; polyamides; polyimides;polystyrene and other vinyl polymers, nylon, ABS and PC-ABS.
 9. Themethod of claim 4 wherein the carbon voile has an approximate density ofless than 10 grams per square meter.
 10. The method of claim 1 wherein ametallic conductive tape or wire is applied along the opposite sides orends of the conductive material for the purpose of applying anelectrical current across the conductive material.
 11. The method ofclaim 10 wherein the conductive tape or wire is removed following thebonding or joint forming process.
 12. The method of claim 1 wherein thevoltage is applied for 10 seconds or less.
 13. A thermoplastic orthermoset polymeric material having a joint or bond formed by the methodof claim
 1. 14. A method for bending a thermoplastic or thermosetpolymeric material comprising: (a) applying a layer of conductivematerial to thermoplastic or thermoset polymeric material; (b)introducing a voltage to the conductive material so as to cause thesurface of the polymeric material to heat to a temperature above itsmelting point and form a bendable portion in the thermoplastic orthermoset polymeric material; (c) bending said polymeric material to adesired angle; and (d) removing said voltage after said desired bendingangle is achieved to allow the thermoplastic or thermoset polymericmaterial to harden at the desired angle.
 15. The method of claim 14wherein the polymeric material is in the form of a sheet.
 16. A methodfor heating a thermoplastic or thermoset polymeric material comprising:(a) applying a layer of conductive material to thermoplastic orthermoset polymeric material; (b) introducing a voltage to theconductive material so as to cause the surface of the polymeric materialto heat to a temperature above its melting point; and (c) removing saidvoltage after heating is no longer desired.
 17. The method of claim 16wherein the polymeric material is in the form of a pipe or a planter.18. The method of claim 16 wherein the polymeric material is in the formof a wall radiator.